Quantum dots, which are spherical semiconductor nanocrystals, are of considerable current interest due to their unique size-dependent properties that are not available from either discrete atoms or bulk solids (Alivisatos, J. Phys. Chem. 100: 13226-13239 (1996); Nirmal et al., Acc. Chem. Res. 32: 407-414 (1999); and Eychmüller, J. Phys. Chem. B 32:104:6514-6528 (2000)). Recent research has demonstrated the wide spectral ranges over which the photoluminescence (PL) of various nanocrystalline materials can be tuned simply by changing the particle size (Murray et al., J. Am. Chem. Soc. 115: 8706-8715 (1993); Hines et al., J. Phys. Chem. 100: 468-471 (1996); Mićić et al., J. Phys. Chem. 101: 4904-4912 (1997); Harrison et al., J. Mater. Chem. 9: 2721-2722 (1999); and Talapin et al., J. Phys. Chem. B 105: 2260-2263 (2001)). Other properties of interest are high quantum efficiencies, narrow and symmetric emission profiles, wide optical absorption bands, and large molar absorptivities. Furthermore, several groups have shown that these highly luminescent nanocrystals can be conjugated to biological molecules such as proteins and nucleic acids for multicolor biolabeling and biosensing (Bruchez et al., Science 281: 2013-2016 (1998); Chan et al., Science 281: 2016-2018 (1998); Mitchell et al., J. Am. Chem. Soc. 121: 8122-8123 (1999), Mattoussi et al., J. Am. Chem. Soc. 122: 12142-12150 (2000); Pathak et al., J. Am. Chem. Soc. 123: 4103-4104 (2001); Dubertret et al., Science 298: 1759-1762 (2002); Jaiswal et al., Nat. Biotechnol. 21: 47-51 (2003); Wu et al., Nat. Biotechnol. 21: 41-46 (2003); Åkerman et al., Proc. Natl. Acad. Sci. USA 99: 12617-12621 (2002); and Murphy, Anal. Chem. 74: 520A-526A (2002)). However, current studies are primarily based on binary semiconductor materials where the emission wavelength is tuned by changing the particle size from about 1 nm to 8 nm. As a result, the largest nanocrystals are expected to have 512 times the volume and 64 times the surface area of the smallest particles. These large differences could cause major problems in bioconjugation and surface chemistry, as well as in the binding and reaction kinetics of nanocrystals to target molecules.
Korgel et al. overcomes some of these problems by generating a series of quantum dots comprising an alloy of ZnyCd1-yS or HgyCd1-yS that, within each series, are fixed in size and composition-tunable (Korgel et al., Langmuir 16: 3588-3594 (2000)). However, each of the quantum dots has a band gap energy that is linearly related to the molar ratio of the semiconductors comprising the quantum dots. The optical properties of these quantum dots, therefore, are still limited in that the range of emission peak wavelengths of the series of the quantum dots is confined to the range of wavelengths defined by the corresponding pure, non-alloyed semiconductor quantum dots, i.e., by the quantum dots consisting of pure HgS, pure CdS, or pure ZnS. Therefore, improved quantum dots comprising an alloy of semiconductors and having unique optical properties that are not limited to the emission peak wavelength range set by the pure, non-alloyed forms are needed in the art.
The invention provides such improved quantum dots, as well as series related thereto, methods of producing either of the foregoing and methods of using either of the foregoing. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.